Reconfigurable intelligent surfaces (RISs) consist of controllable elements that can alter the characteristics of an electromagnetic (EM) wave, such as phase, amplitude, frequency, or polarization [1]}, [2]}, [3]}. By properly configuring the element coefficients, RISs provide controllable communication channels to support communication and localization services for wireless devices [4]}. With the properties of being energy-efficient, lightweight, easy to deploy, and compatible with the existing wireless infrastructures, RISs are expected to be the vital enabler for the sixth generation (6G) communication systems [5]}, [6]}.
